1. Field Of The Invention
The present invention relates to an unobvious process for the preparation of certain known benzotrichlorides chlorinated in the benzene ring which can be used, inter alia, as starting materials for the synthesis of herbicidally active compounds.
2. Discussion Of The Prior Art
The preparation of nuclear-chlorinated aromatic compounds by the reaction of aromatic substances with chlorine is one of the most frequently carried out reactions in organic chemistry. In general, mixtures of various highly chlorinated products and/or of position isomers are thereby obtained, from which pure compounds can be isolated only with a relatively high expenditure of work and in relatively low yields. However, methods which make the controlled syntheses of particular nuclear-chlorinated aromatic compounds possible have also been described in the literature.
Thus, several processes for the selective preparation of nuclear-chlorinated benzotrichlorides have already been disclosed (see U.S. Pat. Nos. 2,608,532, 2,608,591 and 3,297,771, British Patent Specification 771,416 and German Auslegeschrift (German Published Specification) 1,269,115). The disadvantage of these processes is, however, that the pure reaction products are not always obtained in sufficiently high yields. Furthermore, expensive process steps are frequently necessary, or the required starting materials are not accessible in a simple manner.
3,4-Dichlorobenzotrichloride can be synthesised, for example, by treating 1,4-bis-(trichloromethyl)-2-chlorobenzene with chlorine for about 8 hours at temperatures between 240.degree. C. and 250.degree. C. (see British Patent Specification 771,416). Although the desired product is obtained in good yield by this procedure, the industrial application of this process is hindered by several factors. Thus, the energy required for this reaction is quite considerable because very high reaction temperatures must be maintained over a relatively long period. Moreover, the compound employed as the starting material can be prepared only by a synthesis involving several stages.
It is also known that 2,4,5-trichlorobenzotrichloride can be prepared completely analogously to the above-mentioned synthesis of 3,4-dichlorobenzotrichloride by the action of chlorine on 1,4-bis-(trichloromethyl)-2,5-dichlorobenzene for several hours at temperatures of about 250.degree. C. (see British Patent Specification 771,416). However, this process also has the disadvantages already mentioned.
Furthermore, it has been disclosed that 2,4,5-trichlorobenzotrichloride is also formed in the photochlorination of 2,4,5-trichlorotoluene with liquid chlorine at low temperatures (see U.S. Pat. No. 2,608,532). However, the yield of pure product in this procedure leaves much to be desired. In addition, the reaction must be carried out under pressure and the preparation of the isomer-free starting product involves some expense.
Furthermore, it is known that 2,3,4,5-tetrachlorobenzotrichloride can be synthesised by reacting 1,2,3,4-tetrachlorobenzene with carbon tetrachloride in the presence of aluminum chloride (see U.S. Pat. No. 3,297,771). However, in this procedure the compound is obtained only in a very low yield. Quite apart from this, the preparation of the starting material in a pure condition is relatively troublesome.